Do you by any means leave your window tab for long before replying? Usually while replying post like you need to preview after which it will disappear, I think had witnessed something like this.
I also receive an error message too, but I forgot with the explanation.
It's really annoying when we forgot to copy the text, but we've reload the page, so we will lose all the text that have been written.
A new reply to an old thread/topic will (necro-)bump the old thread back to top of recent activity. This may be totally valid and intentional, but the poster should be aware of this and if the current reply is beneficial to the old thread.
@OP is asking about error message, not asking if it's good or not to bump an old topic, after all it's up to every user whether he want to bump old thread or create new thread. The moderator will take action if someone break the rules.